Studies were initiated to determine the biological effects of gamma irradiation on the snail hosts and miracidia of Schistosoma mansoni, with particular reference to the effects of radiation on the susceptibility of the snail hosts to infection and the infectivity of cercariae produced from irradiated snails and miracidia. Results obtained, thus far, indicate that snails exposed to doses of radiation ranging between 3.5 to 5.0 Krads developed a marked resistance to infection and the difference in their infection rates and those of controls were statistically significant. It was also observed that miracidia exposed to radiation as great as 10.5 Krads behaved in a normal manner; actively swimming about and apparently capable of penetrating snails. However, at doses above 7.5 Krads they failed to produce infection in snails. Infection rates in snails exposed to miracidia irradiated at 3.5 Krads were markedly lower than those of controls. Studies are now in progress to determine if cercariae produced from irradiated miracidia and from snails subject to radiation are infective to the mammalian host.